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First direct observation of Gravitational Waves from a Binary Black Hole Merger

On September 14, 2015 at 11:50 a.m. Central European Time the two detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) simultaneously observed a gravitational-wave signal, shown in Fig 1. This epic and historical discovery was an-nounced last week, on Thursday 11, 2016.

Gravitational waves were first predicted to exist by Albert Einstein one century ago. These waves travel at the speed of light and they are generated by any mass acceleration that is not spherically or cylindrically symmetric. For instance, they can be produced by the merger of two astrophysical objects like black holes or neutron stars. In that very same year 1916, Schwarzschild found the solutions of the Einstein equations, describing a black hole. Further studies and theories have enabled modeling of binary black hole mergers providing accurate predictions for the associated gravitational wave pattern. A century after the theoretical predictions of Einstein and Schwarzschild, LIGO has reported the first direct detection of gravitational waves. From the observed pattern, the LIGO collaboration has inferred it was produced by two black holes, with corresponding masses of approximately 36 and 29 solar masses, releasing, just before their merging, the energy equivalent to three solar masses in the form of gravitational waves, which implies much more energy than all stars and galaxies in the universe where releasing combined. The observation is depicted in Fig. 1. The top panels show the measured signal in the Hanford (top left) and Livingston (top right) detectors. The central panels show the ex-pected signal produced by the merger of two black holes, based on numerical simula-tions. The bottom panels show a time-frequency representation of the strain data, showing the signal frequency increasing over time. Figure 3 shows the interpretation of the observed signal. During the first part of the signal, both black holes rotate around their common center of mass and fall in a spiral towards each other. Their event horizons merge, forming a single black hole, and generating the higher frequency gravitational waves. Eventually, these vibrations stop, and all that is left is a single rotating black hole.

LIGO’s major discovery puts on a solid ground a new emergent branch of observational astronomy — gravitational wave astronomy, which is complementary to the other ways of looking at the universe through light or neutrinos. Other gravitational wave detectors - such as the Virgo interferometer near Pisa, Italy, and the GEO600 interferometer near Hannover, were not operating at the time of LIGO’s observation and theerfore could not confirm the signal. However, these detectors, together with other upcoming Earth-based detectors, such as Advanced Virgo, KAGRA in Japan, and possibly a third LIGO detector in India will further enlarge the global gravitational wave detector network. In the future, eLISA, a space-based interferometer, will enable us to go deeper into the cosmos than ground-based detectors. Gravitational wave detection will allow new and more precise measurements of astro-physical sources. This early discovery made by LIGO means that black hole mergers will be observed quite often with the current version of the LIGO detector (Advanced LIGO), which will be further improved over the next few years. Gravitational wave astronomy will hopefully help us understand more about black holes, neutron stars and other massive astrophysical objects. Perhaps it will also help explaining some unsolved questions in astrophysics, such as the origin of gamma-ray bursts and give us further insight on the evolution of the early universe.